Pull hard-coded Elastic Stack host, port and availability zone configuration out of `write_elastic()`

These values are hard-coded into write_elastic() because it was convenient when I was creating and testing it. They should be pulled out of the function so they're user-configurable for each runtime environment. They could be read from a file like /etc/crawler/config or something like a json or yaml file in the application root directory or provided some other way, like as environment variables sourced from /etc/crawler/secrets.